Until the middle of the first century, 10 cohorts (about 5,000 men) made up a Roman Legion. This was later changed to nine cohorts of standard size (with 6 centuries at 80 men each) and one cohort, the first cohort, of double strength (5 double-strength centuries with 160 men each).

Each legion had 59 centurions, one to command each centuria of the 10 cohorts.

The commanding officer of a Roman legion was called a legate, or Legatus Legionis.

The First Cohort had more soldiers than the other cohorts. The first cohort was made up of 800 men (5 centuries with 160 men each). The other 9 Cohorts had a total of 4,320 soldiers (6 centuries of 80 men each).